Drive system including helper motor



Aug. 22, 1961 F. BENEDITZ 2,996,876

DRIVE SYSTEM INCLUDING HELPER MOTOR Filed Sept. 8, 1959 2 SheetsSheet 1STEAM TURBINE i HELPER DRIVE MOTOR f5 BELT DRIVES H d J (4 DRIVEN 2MACHINE 44 O Volts no VoHs INVENTOR, FRED BENEDITZ am. 1M :4 5. ,0 766%ATTORNEYS Aug. 22, 1961 F. BENEDITZ 2,996,376

DRIVE SYSTEM INCLUDING HELPER MOTOR Filed Sept. 8, 1959 2 Sheets-Sheet 276a T AL :7 :5 i GOVERNOR CLOSED 1 MICRO-SWITCH I3 1:2 GOVERNOR OPEN B2MICRO-SWITCH 2o 1% 1 I i "'1 OPEN MTHROTTLE- GOVERNOR a; POSITION ROD'/4 FBI 5 a: CLOSED 5 m IN VEN TOR.

1 5) 9 GOVERNOR FRED BENEDITZ THROTTLE 8 BY wax/M 4 ATTORNEYS UnitedStates Patent Patented Aug. 22, 1961 2,996,876 DRIVE SYSTEM INCLUDINGHELPER MOTOR Fred Beneditz, Tomahawk, Wis., assignor to Owens- IllinoisGlass Company, a corporation of Ohio Filed Sept. 8, 1959, Ser. No.838,651 9 Claims. (Cl. 60--6) This invention relates to drive systemswherein a prime mover drives a load member, such as a machine having arotary member which is to be driven by the prime mover. Moreparticularly, this invention relates to such a drive system whichincludes a helper motor capable of assisting the prime mover to permitthe latter to operate at less than maximum capacity and thereby have theability to compensate for variations in the load to be driven.

When a prime mover, such as a steam turbine, is driving a load whichrequires the prime mover to operate at very nearly maximum capacity, itcannot compensate readily for fluctuations in the load which cause theprime mover to attempt to exceed its maximum capacity. Thus, if amachine is being driven by a steam turbine and the power transmissionarrangement between the turbine and machine is such that to drive themachine at the desired speed requires the turbine to operate at verynearly full throttle, fluctuations in the load or in the steam pressurecannot be compensated for by the turbine governor since the throttlecannot be opened further. It is apparent that, under such anarrangement, the turbine may stall, with the result that the drivefails.

The present invention is directed toward the problem of providing adrive system wherein the prime mover, a steam turbine for example, ispermitted to operate within a predetermined range which is well withinits capacity. This is accomplished by providing a helper drive motorwhich supplies just enough power to permit the prime mover to operatewithin the predetermined range mentioned. The prime mover is used as themain source of power and power from the helper drive is used only whenhelp is needed. Moreover, according to the present invention, the helperdrive always remains a slave to the prime mover and its controls and, tothis end, a drive system according to the present invention includes asan important feature thereof an automatic control system which makes thesystem versatile and guards the system against reasonably predictablemalfunctions as will be more evident from the detailed description whichfollows.

It is therefore an object of the present invention to provide a new andimproved system for driving a load member by means of a prime moverwherein the system includes a helper drive which permits the prime moverto operate in a range which is well within the capacity of the primemover.

It is another object of the present invention to provide such a systemwherein the helper drive always remains a slave to the prime mover andits controls.

It is a further object of the present invention to provide such a systemwhich is versatile in that the amount of load carried by the helperdrive is readily adjustable.

It is still another object of the present invention to provide such asystem which includes automatic controls which not only insure that thehelper drive is versatile and remains a slave to the prime mover and itscontrols, but also guard against reasonably predictable malfunctions ofthe system.

Briefly described, a preferred embodiment of a system according to thepresent invention comprises a steam turbine (:as the prime mover) which,through a suitable gear reducer and belt drive power transmissionarrangement, drives a load machine wherein the load imposed on the primemover may fluctuate for any particular speed at which the load machineis driven and, of course, can vary according to the speed at which theload machine is driven. A three-phase wound rotor slip ring inductionmotor has its output shaft belt coupled to the transmission arrangementso that the electric motor can assist the ourbine to drive the load.This permits the turbine to operate at less than maximum capacity andthereby have the ability to compensate for variations in load.

The helper drive includes the drive motor itself, motor control grids,and an automatic control system to coordinate the electric drive withthe steam turbine. Motor torque is varied with a motorized drum selectorswitch and a set of grid resistors. The automatic control systemincludes an overcurrent type of relay which prevents an overload trip ofthe helper drive should the control system demand more torque than thedrive is capable of producing. When the maximum permissible line currentis reached, the control will not respond to a demand for increase drive,and should excess current flow, the control will unload the helper driveto the point of maximum allowable current. Power interruption will causethe drive to fail but it will return to service automatically with therestoration of power. On its return, the helper drive will step up fromzero torque to its normal load, removing the possibility of overdrivingthe turbine before the control has had time to correct for load changeswhich may have occurred.

A pair of control switches, operatively associated with the turbine,determine when the automatic control goes into action to either increaseor decrease the amount of torque supplied by the electric helper drivemotor. These switches are adjustable to permit the turbine to operatewithin a predetermined range below its maximum capacity. This means thatthe turbine can respond to fluctuations in the load, and the helperdrive comes into action to either increase or decrease its output onlywhen the load fluctuation, or turbine speed, causes either controlswitch to be actuated to bring the helper drive into play.

Qther objects and advantages of the present invention will become moreapparent from the following detailed description, taken in conjunctionwith the attached drawings, in which:

FIG. 1 is a diagram of a drive system according to an embodiment of thepresent invention;

FIG. 2 is an enlarged detail view showing control members forcontrolling increase and decrease of the amount of torque supplied bythe helper drive;

FIG. 3 is an electrical circuit diagram of the automatic control circuitfor he helper drive motor; and

FIG. 4 is an enlarged circuit diagram of the helper motor external rotorcircuit shown in FIG. 3.

Referring to FIG. 1, the block designated by reference numeral 1represents a prime mover which, according to a preferred embodiment ofthe invention, is a steam turbine. The block designated by referencenumeral 2 represents a machine to be driven by turbine 1. The drive isaccomplished through a suitable power transmission arrangement includinga gear reducer (designated by block 3), and a belt drive 4. Of course,gear reducer 3 and belt drive 4 are simply exemplary of a type of powertransmission means which may be employed between turbine 1 and drivenmachine 2.

A helper drive motor, designated by block 5, has its output coupled tothe output shaft 6 of gear reducer 3 by another belt drive 7. By thisarrangement, the helper drive motor can supply power to shaft 6 and thusassist turbine 1 in driving the load, or driven machine, 2.

Referring now to FIG. 2, and still using a steam turbine as the primemover 1, it is seen that the turbine throttle 8 and turbine governor 9are both operatively coupled to a throttle-governor position rod 10. Thearrangement is such, as will be appreciated by those skilled in the art,that rod 10 is moved in accordance with movements of the turbinethrottle and governor whereby, as

seen in FIG. 2, the rod will rise as the throttle and governor open andwill fall as they close. Rod is shown as having a pointer 11 connectedthereto which moves along a scale 12 to indicate the extent to which thethrottle and governor are open.

Two collars 13 and 14 are positioned on rod 10 as shown in FIG. 2. Twoswitches 15 and 16 (microswitches, preferably) are supported withrespect to rod 10 and collars 13 and 14 by suitable means, not shown, sothat each switch is capable of being actuated by a collar. Thus, switch15 has a roller 17 carried by an arm 18 projecting from the switch, androller 17 is adapted to be engaged by collar 13 (as shown in FIG. 2)whereby the roller and arm are moved in a manner to actuate switch 15.As described hereinafter, such actuation results in closing of switch15. Roller 19 and arm 20 are operatively associated with switch 16. Asshown in FIG. 2, roller 19 is not contacted by either collar 13 or 14,with the result that switch 16 is not actuated when rod 10 is in theposition shown in FIG. 2.

The spacing between collars 13 and 14, and the spacing between switches15 and 16, both spacings being adjustable as desired, are chosenaccording to a preferred embodiment of the present invention so thatneither switch 15 or 16 is actuated (closed) when the turbine isoperating in the range from one-half to three-quarters capacity, i.e.when the turbine throttle is from one-half to threequarters open. Thus,referring to FIG. 2, the spacing is such that when rod 10 rises to anextent such that pointer 11 reaches the one-half mark on scale 12,roller 17 will roll off of collar 13 and open switch 15. If rod 10continues to rise, as when throttle 8 is opened further, collar 14 willcontact roller 19 to close switch 16 when pointer 11 passes thethree-quarter mark on scale 12. As is explained in greater detailhereinafter, when switch 15 is closed the torque supplied by helperdrive motor 5 is decreased, and when switch 16 is closed, the torque isincreased. When neither switch is closed, the torque supplied by thehelper motor remains substantially constant with turbine 1 compensatingfor fluctuations in the load. The helper drive motor thus remains aslave to the prime mover and its controls and the prime mover thusperforms its intended function of being the prime source of power forthe driven machine 2.

Referring now to FIG. 3, helper drive motor 5 is shown as being a woundrotor slip ring induction motor having a stator energizable fromthree-phase power lines 21 when magnetic starter relay coil 22 isenergized as described in greater detail hereinafter. The external rotorcircuit of motor 5 includes a variable resistance grid assemblydesignated generally by the reference numeral 23 and whereby the totalresistance of the rotor of motor 5 may be varied in order to vary theoutput torque of motor 5. As will be appreciated by those skilled in theart, a wound rotor induction motor is capable of producing a relativelyconstant torque over a wide range of sp ed so that the output torque issubstantially independent of the motor speed but is controllable by anexternal resistance in the rotor circuit. For a given resistance, themotor produces almost constant torque while following the speed of theturbine (or other prime mover) to which it is coupled. This is adesirable situation since the turbine is the prime mover and itsthrottle should determine the speed of the driven machine.

Resistance grid assembly 23 includes three electrically conductivecontact arms 24, 25 and 26 which, as shown in FIG. 4, project from acommon hub 27 so that when the hub is turned each arm will be moved. Hub27 is driven by a reversible control motor 28 through a suitableconnection indicated generally by dotted line 29. The arrangement issuch that the output of motor 28 is stepped down appreciably wherebyarms 24-26 will be turned slowly to add or subtract resistance smoothlyand gradually in the rotor circuit of helper motor 5.

Reversible control motor 28 has one side connected to lead 30, and theother side selectively connectible to either lead 31, through limitswitch 32 and adjustable resistance 33, or lead 34, through limit switch35 and adjustable resistance 36. As is explained in greater detailhereinafter, control motor 28 is connected to lead 34 when it is desiredto decrease the resistance in the rotor circuit of helper motor 5, andcontrol motor 28 is connected to lead 31 when it is desired to increasesuch resistance.

Control motor lead 30 is connected to one side (the grounded side) 37 ofa suitable power source which has lead 38 as its other side. The poweracross leads 37 and 38 may be, for example, volt AC. power. Lead 38extends, via lead 39, to a contact 40 of torque decrease relay 41, and acontact 42 of torque increase relay 43. When relay 41 is energized, itpicks up contact arm 44 to complete a circuit from power lead 38 tocontrol motor lead 31 via thermal overload device 45. Similarly, whenrelay 43 is energized, it picks up contact arm 46 to complete a circuitfrom power lead 38 to control motor lead 34, via thermal overload device47.

Power lead 38 goes to what can be described as a master control switch48 shown in FIG. 3 as being in open position. When switch 48 is closed,power lead 38 is connected to a lead 49, through torque adjust switch50, and is also connected to a lead 51, through normally closed contactdevices 52 and 53. Device 52 may be, for example, an overspeed trip onturbine 1, and device 53 may be a thermostat trip operatively associatedwith resistance. grid assembly 23 so that if the grids should overheatdevice 53 will open to disconnect other components of the overallcontrol circuit from power lead 38.

Lead 49 goes to contacts 54 and 55 of control relay 56. This controlrelay includes three contact arms 57, 58 and 59, one of which (57) is inclosed position when the relay is deenergized, and the other two ofwhich are in open position when the relay is deenergized. With contactarm 57 thus being in closed position, and with switch 50 closed, it isevident that when master switch 48 is closed power lead 38 is conectedthrough to a lead 60 which goes, through thermal overload device 45, toone side of torque decrease relay 41. The other side of relay 41 (andalso relay 43) is connected to grounded lead 61.

Referring now to the upper right hand portion of FIG. 3, two switches 62and 63 are shown as connected in series between one side of normallyclosed contact device 53 and one side of control relay 56. Switch 62 isa manually operable restart switch. Switch 63 is a switch located in theresistance grid assembly 23 (see FIG. 4) and is open except when theresistance grid assembly contact arms 2426 are in the zero torqueposition as is explained more fully hereinafter.

Further detailed description of other components shown in FIGS. 3 and 4is presented in connection with a description of operation of thecricuitry shown in FIGS. 3 and 4 which follows.

In starting this description of operation, it is assumed that turbine 1is running with its throttle 8 less than onehalf open, that the circuitof FIG. 3 is deenergized except that power is available at leads 21, and37, 38, and that the contact arms 2426 of resistance grid assembly 23are out of the zero torque position and in some other position betweenthe permissible end limits of their movements. This is the situation asshown in FIG. 3.

Now, if master switch 48 is closed, power is supplied from lead 38through switches 48, 50, lead 49, relay contact arm 57, and lead 60 toone side of decrease torque relay coil 41 to energize this coil. Whenenergized, coil 41 picks up contact arm44 to connect lead 31 of controlmotor 28 to power lead 38 via thermal overloaddevice 45, contact arm 44,and lead 39. Since the other side of control motor 28 is connected, vialead 30, to the grounded lead 37 of the power source, control motor28is'energized. When so energized, it act uates hub 27-to turn contact arms24-26 in a counterclockwise"direction'as viewed in FIGS. 3 and 4, untilthese contact arms reach the zero torque position represented by thedotted line showing of these arms (24a-26a) as seen in FIG. 4.

When sliding contact arms 2426 are in this zero torque position, switch63 is closed and, in the example shown, switch 63 is shown as beingoperatively associated with contact arm 24. Details of the mountingarrangement whereby switch 63 is adapted to be closed when arms 2426 arein the zero torque position, and is adapted to open when these armsleave this position are omitted since it is believed evident to a personskilled in the art that any suitable mounting arrangement may beutilized to provide the action desired. In FIG. 4, reference numerals 64and 65 are shown as applied to leads extending to switch 63, and thesereference numerals are also applied in FIG. 3 to correlate FIGS. 3 and4.

When switch 63 closes, power lead 38 is connected through to one side ofcontrol relay 56 which energizes this relay since the other side of therelay is connected to grounded power lead 37 as shown in FIG. 3.Indicator light 66 goes on to show that the control relay is energized.When energized, relay 56 opens contact arm 57 and closes contact arms 58and 59. Contact arm 59 completes a circuit across one phase of thethree-phase power leads 21 (as shown in FIG. 3) to energize magneticstai'ter relay 22. This closes magnetic starter contact arms 67 and thusenergizes the stator of wound rotor induction motor 5. A fan motor 68 isalso energized and this motor drives a fan for cooling the resistancegrids of resistance grid assembly 23.

Magnetic starter relay 22 includes a fourth contact arm 69 which is alsoclosed when the relay is energized to provide a circuit via lead 51,contact arm 69, and lead 70 to one side of control relay 56. As can beseen from FIG. 3 this seal or holding circuit bypasses switches 62 and63 so that control relay 56 can remain energized even though eitherswitch is opened as occurs when the movable contact arms 2426 leave thezero torque position. However, while the seal or holding circuit is thusset up as described, switch 63 is not opened until control motor 28 isenergized to cause switch 63 to be opened and, therefore, even thoughthree-phase power is applied to the stator of helper motor 5, this motordoes not deliver any output torque because its rotor circuit is open atthis time.

On the assumption that turbine 1 is operating at less than halfthrottle, micro-switch 15 will be closed and micro-switch 16 will beopen as has been described above. This condition is shown in FIG. 3where the closed micro-switch 15 is shown as being located between relay41 and contact 72 of control relay 56 whereby, when control relaycontact arm 58 close and arm 57 opens, power can still be supplied toone side of torque decrease relay 41. Micro-switch 16 is shown as beinglocated in a line 71 which comes from control relay contact 72, goesthrough a normally closed contact arm 73 of a modified overcurrent relay74 (described in greater detail hereinafter) and goes throughmicro-switch 16 (when the switch is closed) to one side of torqueincrease relay 43.

When the turbine throttle 8 goes past half-open position, and before itreaches three-quarters-open position, micro-switch 15 opens, andmicro-switch 16 remains open, as has been described above. The openingof microswitch 15 makes no difference at this stage of the operationbeing described since contact arms 2426 of resistance grid assembly 23have previously been moved to the above-described zero torque positionand are still located in this position. However, should turbine throttle8 go past three-quarter-open position then, as has been described above,micro-switch 16 is closed by collar 14 on the throttle-governor positionrod 10.

Closing of micro-switch 16 connects one side of torque increase relay 43to power lead 38 through switches 48 and 50, lead 49, and contact arms58 and 73. When relay 43 is energized, its contact arm 46 is closed toconnect lead 34 of control motor 28 to power lead 38 via lead 39,contact arm 46, and thermal overload device 47. The internal circuit ofcontrol motor 28 is such that when this motor is energized as justdescribed, it turns movable contact arms 2426 in a clockwise directionas viewed in FIGS. 3 and 4 to first open switch 63, then connect themaximum resistance into each phase of the helper motor rotor circuit,and then reduce the resistance in each phase as movement of the contactarms continues. As will be appreciated by those skilled in the art,reduction in the resistance of the external rotor circuit of helpermotor 5 results in an increase in the output torque of the helper motorwhereby the helper motor assumes a share of the load being driven by theturbine and thus assists the turbine in driving the load. It i to benoted furthermore that, by having the helper motor start from zerotorque, i.e., with its rotor circuit open, and then going from fullresistance in the rotor circuit to a decreased resistance in the rotorcircuit, the helper motor gradually assumes a share of the load andtherefore does not come into action with an output torque which mightprovide too much of a jolt on the system.

As the helper motor increases its share of the load, turbine throttle 8will ultimately drop back below threequanter-open position. When it doesso drop back, throttle-governor position rod will likewise move untilcollar 14 no longer engages roller 19 on actuating arm 20 ofmicro-switch 16, whereupon the micro-switch will open to deenergizecontrol motor 28. This stabilizes the system with both the turbine andthe helper motor sharing the load. If the turbine throttle closes toless than half-open, micro-switch 15 will again be closed and energizetorque decrease relay 41 to put more resistance in the rotor circuit ofthe helper drive motor.

Referring more specifically to FIG. 4, it is noted that each of theresistance elements 75 which is adapted to be connected in a phase ofthe helper motor rotor circuit is actually made up of a plurality ofseries-connected resistance elements 76 to which contact segments 77 areconnected so that, as the movable contact arms 2426 travel over thecontact segments, resistance will be removed from or added to the helpermotor rotor circuit in predetermined increments. It is to be notedfurther more that the positioning of contact arm 24 relative to contactsegment 77a, the positioning of contact arm 26 relative to contactsegment 77b, and the positioning of contact arm 25 relative to contactsegment 770, are such that, as the contact arms move slowly in aclockwise direction as viewed in FIG. 4, contact arm 24 will contactsegrnent 77a first, then contact arm 26 will contact segment 77b, andfinally contact arm 25 will contact segment 770. When arms 24 and 26contact segments 77a and 77b, this completes a single-phase connectionof the helper drive motor rotor circuit. When contact arm 25 contactssegment 77c, this completes a three-phase connection of the rotorcircuit and, as movement of the contact arms 2426 continues, aresistance element 76 is removed one phase at a time so that outputtorque changes are quite smooth.

Dot-dash lines 24b-26b represent the upper limits of movement of movablecontact arms 2426. When the contact arms reach their upper limit ofmovement, they open a limit switch 35 which, as can be seen from FIG. 3,deenergizes control motor 28. Lead designations 78 and 79 are shown inFIGS. 3 and 4 in order to correlate the position of limit switch 34 inthese two figures. When contact arms 2426 reach their lower limit ofmovement (represented by dotted lines 24a-26a), they open limit switch32 to deenergize control motor 28 and prevent a further attempteddecrease of the torque supplied by helper drive motor 5. Referencenumbers 80 and 81 have been applied to leads shown in FIG. 4 in order tocorrelate this figure with FIG. 3 insofar as limit switch 32 isconcerned. It is to be noted that even when the contact arms 2426 are attheir upper limit of movement, the resistance elements 76a-76c are stillleft in the rotor on cuit so that there is not a complete short circuitacross the rings insofar as the resistance grid assembly 23 isconcerned.

The movable contact arms 24-26 and contact segments 77 may be made ofany suitable conductive material sufficiently strong for the purposeintended. Also, as shown in FIGS. 3 and 4, the common conductive hub 27to which contact arms are attached is grounded, and each of the threeresistances 75 is connected to a helper drive motor rotor'slip ring 82.Of course, the number of resistance elements 76 shown [in FIG. 4 issimply exemplary, it being understood that any suitable arrangement ofresistances can be provided depending upon the desired smoothness of thetorque increase or decrease.

A further feature of the control circuit shown in FIG. 3 is overcurrentrelay 74 which functions to prevent the helper drive motor from beingoverloaded and kicking out the thermal overloads 83 which, as shown inFIG. 3, are located in the power lines leading to the helper motorstator circuit. Relay 74 is connected to a current transformer 84through an ammeter 85, said current transformer being coupled to one ofthe stator input lines of the helper motor as shown in FIG. 3. When thehelper drive motor 5 is drawing almost full load current, relay 74operates to move contact arm 73 away from contacts 86 and 87 but not farenough to bridge contacts 88 and 89. This breaks the circuit from powerlead 38 to increase torque relay 43, even though micro-switch 16 isstill closed, thus deenergizing control motor 28 and preventing thecontrol motor from taking any more resistance out of the rotor circuitof helper motor 5.

Should the current drawn by the helper drive motor continue to rise,relay 74 causes contact arm 73 to bridge relay contacts 88 and 89. Thiscompletes a circuit from power lead 38 through to decrease torque relay41, and energizes relay 41 to, in turn, energize control motor 28 to addresistance to the rotor circuit of helper drive motor 5. Contact arm 73will bridge relay contacts 88 and 89 if the load imposed by drivenmachine 2 becomes too great with both turbine 1 and helper drive motor 5delivering maximum power. If the load thus does hecome too great, thespeed at which the load is driven will start to decrease. Moreover, whenthe output torque of the helper drive motor 5 is reduced as aconsequence of contact arm 73 bridging relay contacts 88 and 89, thespeed will continue to drop more rapidly, resulting in a stall whichwill cause the usual turbine protective system to go into action toprotect the turbine. Meanwhile, overcurrent relay 74 has effectivelyprevented the helper drive motor from being overloaded and it isapparent that the action of overcurrent relay 74 is predetermined sothat contact arm 73 does bridge contacts 88 and 89 before the statorcurrent becomes sufiicient to trip the thermal overloads 83.

From the foregoing detailed description, it is seen that a drive systemaccording to the present invention does not simply include a helperdrive motor which assists a prime mover to drive a load but, instead,includes a helper drive motor in such a way that the helper drive alwaysremains a slave to the prime mover and its controls, and the controlswitches and 16) for the helper drive are completely adjustable as towhen the helper drive will come in and what percentage of the load itwill assume. The system is thus versatile and, additionally, it guardsagainst any reasonably predictable malfunction.

Any type of turbine failure will remove the helper drive from the line,as will any failure of the helper drive itself. Power interruption willnaturally cause the helper drive to fail but it will return to serviceautomatically with the restoration of power. On its return, the helperdrive will step up from zero torque to its normal load, removing thepossibility of overdriving the turbine before the control has had timeto correct for load changes which may have occurred. In the event of aturbine overspeed trip, the helper drive will drop out instantly. Whenthe turbine is returned to service, the drive will again startautomatically.

While I have described and illustrated a preferred embodiment of myinvention, I wish it to be understood that I do not intend to berestricted solely thereto but that I do intend to cover allmodifications thereof which will be apparent to one skilled in the artand which come within the spirit and scope of my invention.

What is claimed is:

1. A drive system comprising a prime mover having an adjustable poweroutput, a load member, power transmitting means connecting said loadmember in driven relation to said prime mover, a helper drive motorhaving an adjustable power output, means connecting said helper motor tosaid power transmitting means so that said helper motor can help saidprime mover to drive said load member, a first control member, a secondcontrol member, means connecting both said first and second controlmembers to said prime mover, said connecting means being operable toactuate said first control member in response to one predetermined valueof power output from said prime mover and being operable to actuate saidsecond control member in response to another predetermined value ofpower output from said prime mover, and said connecting means beinginoperable to actuate either control member throughout a range of poweroutputs from said prime mover lying between said predetermined values ofsaid power output, said range being a substantial fraction of the entirerange of power outputs from said prime mover, helper motor control meansoperable when actuated to vary the power output of said helper motor,and means connecting both said first control member and said secondcontrol member to said helper motor control means, said helper motorcontrol means being operable in response to actuation of said firstcontrol member to increase the power output of said helper motor, andbeing operable in response to actuation of said second control member todecrease the power output of said helper motor.

2. A drive system according to claim 1, wherein said means connectingsaid control members to said helper motor control means includes meansoperable in response to actuation of said first control member tooperate said helper motor control means to first decrease and thenincrease the power output of said helper motor.

3. A drive system comprising a prime mover having an adjustable poweroutput, a load member, power transmitting means connecting said loadmember in driven relation to said prime mover, an electrical helperdrive motor having an adjustable power output, means connecting saidhelper motor to said power transmitting means so that said helper motorcan help said prime mover to drive said load member, a first controlswitch, a second control switch, means on said prime mover engageablewith both of said switches, said prime mover means being operable toactuate said first control switch in response to one predetermined valueof power output from said prime mover and being operable to actuate saidsecond control switch in response to another predetermined value ofpower output from said prime mover, a helper motor control operable whenenergized to vary the power output of said helper motor, meansconnecting said control motor to said helper motor to vary the poweroutput of said helper motor when said control motor is energized, andelectrical circuit means electrically connecting both of said controlswitches to said helper motor control motor, said electrical circuitmeans including means operable in response to actuation of said firstcontrol switch to energize said control motor to increase the poweroutput of said helper motor, and also including means operable inresponse to actuation of said second control switch to energize saidcontrol motor to decrease the power output of said helper motor.

4. A drive system comprising a prime mover having an adjustable poweroutput, a load member, power transmitting means connecting said loadmember in driven relation to said prime mover, an electrical helperdrive motor having a stator circuit and a rotor circuit, said rotorcircuit including an adjustable impedance, means connecting said helpermotor to said power transmitting means so that said helper motor canhelp said prime mover to drive said load member, a pair of controlswitches, means'on said prime mover engageable with each control switchto actuate each control switch, said prime mover means being operable toactuate one of said control switches in response to one adiustablypredetermined value of power output from said prime mover and beingoperable to actuate the other control switch in response to anotheradjustably predetermined value of power output from said prime mover, areversible electrical control motor, means connecting said control motorto the variable impedance in the rotor circuit of said helper motor,said control motor being operable, when energized, to either increase ordecrease said impedance, and electrical circuit means connecting bothcontrol switches to said reversible control motor, said circuit meansincluding means operable in response to actuation of said first controlswitch to energize said control motor to decrease said rotor impedanceand thereby increase the output torque of said helper motor, and alsoincluding means operable in response to actuation of said second controlswitch to energize said control motor to increase said rotor impedanceand thereby decrease the output torque of said helper motor.

5. A drive system according to claim 4, wherein said helper motor is athree-phase wound rotor induction motor, and said adjustable impedanceincludes an adjustable resistance located in each phase of said rotorcircuit.

6. A drive system according to claim 4, wherein said adjustableimpedance in the rotor circuit of said helper drive motor includes meansoperable, when actuated, to open said rotor circuit.

7. A drive system according to claim 4, wherein said electrical circuitmeans includes means operable, when actuated, to prevent any furtherdecrease of the impedance in the rotor circuit of said helper motor whenthe stator circuit of said helper motor is drawing substantially fullload current.

8. A drive system according to claim 7, wherein said electrical circuitmeans includes means operable, when actuated, to energize said controlmotor to increase the impedance in the rotor circuit of the helper motorin the event that the stator circuit of the helper motor continues todraw substantially full load current and the driven load ultimatelybecomes too great for both the prime mover and the helper motor.

9. A drive system according to claim 6, including means for energizingthe stator circuit of said helper motor prior to closing the rotorcircuit of said helper motor.

References Cited in the file of this patent UNITED STATES PATENTS1,670,070 Holt May 15, 1928 1,943,369 Coates Jan. 16, 1934 2,328,451Hedman Aug. 31, 1943 2,466,358 Besserdick et a1. Apr. 5, 1949 2,871,660McDonald et al Feb. 3, 1959

